Mrs May chaired an emergency Brexit meeting of the Cabinet in Downing Street amid signs of rebellions from within the Tories.
MPs have raised fears that her proposed deal with the EU for Britains exit is the not what people voted for in June, 2016.
She is due to make a statement outside Downing Street tonight after the marathon session of Cabinet.
The Prime Minister is making a final bid for consensus on the deal – which was heavily criticised today by both sides of the House of Commons.
Outgoing cabinet minister Jo Johnson described Mrs Mays plans as “vassalage” and it was called a "middle muddle fiddle fuddle" by Brexiteer MP Jacob Rees-Mogg.
With Britain still bitterly divided over Brexit, the PM is staking her future in the top job on pushing through her deal.
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“I am confident that this takes us significantly closer to delivering what the British people voted for in the referendum”
Theresa May
Mrs May's plans for Brexit have already seen a slew of ministers resign over the past year – including top Brexiteers former foreign secretary Boris Johnson and Brexit secretary David Davis.
Speculation is now growing that international development secretary Penny Mordaunt, and the secretary for work and pensions Esther McVey will resign today over Brexit.
The Prime Minister was originally due to make a statement at 5pm, but the Cabinet meeting is now overunning – with reporters gathered outside Number 10.
BBC political editor Laura Kuenssberg said: "Cabinet running maybe as much as two hours over. Longer than expected Cabinet meetings dont normally bode well."
CRUNCH TIME: Theresa May is leading a marathon session of the Cabinet to thrash out Brexit (Pic: GETTY)
Mrs May took a battering in Prime Ministers Questions as she laid out plans for her deal ahead of the crunch meeting with Cabinet.
She said: "The Cabinet will decide on the next steps in the national interest.
"I am confident that this takes us significantly closer to delivering what the British people voted for in the referendum."
Labour leader Jeremy Corbyn said the draft deal was "a failure in its own terms" and a senior spokesman said that the "likelihood" was that the party would vote against it in the Commons.
DIVIDED: Protestors from both Leave and Remain camps descended on the House of Commons (Pic: GETTY)
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FURY: Brexit supporters have accused Theresa May of carrying out the will of the people (Pic: GETTY)
Sky News reports a Downing Street source said: "Everyone wants their say and they are talking for longer than normal.
"Hugely important moment. Everyone wants their say."
The source added the PM as answering points as they go along – rather than the usual format of ministers saying their piece before the PM sums up.
Mrs May's coalition partners the DUP have also warned there will be "consequences" if the Brexit deal does not protect Northern Ireland.
SPLIT: Theresa May will have to get Cabinet to agree her deal – before taking it to the Commons (Pic: GETTY)
Former Prime Minister Tony Blair savaged Mrs May's Brexit deal, saying it amounted to "capitulation" to the European Union.
The former prime minister, a prominent Remain campaigner, said the plan would give the European Union a veto over divergence from Brussels' rules.
He urged Labour MPs to reject the deal and back a second referendum, hitting out at Jeremy Corbyn's "abject refusal" to "lead the country out of the Brexit nightmare".
In a speech in London, Mr Blair said he had some sympathy with the Prime Minister who was faced with "an impossible circle to square" in keeping a frictionless border with Ireland but extracting the UK from the single market and customs union.
BREXIT: Britain remains divided over leaving the EU – with the referendum split 52% to 48% (Pic: GETTY)
The Prime Minister will require a simple majority of the 650 MPs to vote in favour of her deal when it is put before Parliament.
But this will prove anything but simple, and whether she can get enough MPs into the aye lobby is now arguably the biggest question in Brexit politics.
Brexit has left both the Tories and Labour divided between those who support leaving and those who do not, and various nuanced factions in between.
While there are 650 MPs, seven Sinn Fein MPs do not take their Westminster seats and the Speaker and his three deputies do not vote.
So the magic number Theresa May needs to meet or surpass is 320 – ie more than half of 639.
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